I was one of the Ello devs (mobile, then web, then backend, then all of it).

My guess is that you'll have some diehard fans early on - Ello did, and they stayed to the end (thanks!) and they'll bemoan every single change - just roll with it, trust yourself but also listen to them.

If you get some success you will have a spammer problem (esp porn), but if you limit (better: don't have) public feeds maybe you can avoid the worst of this. Mutual follow makes sense as a way to limit the blast radius of bad actors, but then people will get spammed by friend requests. It SUCKS. Maybe you punish accounts that blast lots of requests but get very few follows back. We did something like this.

Actually one idea that I think would work well: once you have a "this is a spam account" determination, hide those spam accounts from real users, but let them engage with other spam accounts. This will cost you some compute time, but it will give them the feeling that they are being successful without distracting users.

Good luck!